1. Field of the Invention
This invention relates to telephone station apparatus.
Recent years, especially in business communications, have seen the advent of the loudspeaker telephone terminal by means of which several conferees can speak and listen, at one location, to other conferees at a remote location. A hybrid of this and conventional person-to-person calling is the group listening system. In this system, one of an assembled group of people has access to a telephone permitting telephone conversation with a remote party, the telephone being connected to a loudspeaker which, when switched on, emits an amplified version of the telephone receiver output. This enables one of the group to interact with the remote party while the rest of the group are passive listeners. It is a useful service in, for example, a stockbroking environment in which one share dealer might be negotiating a particular deal, and other dealers, although not active parties in the deal, need to know directly if the deal is consummated since it might have appreciable impact on the subsequent price of the relevant share.
A problem arises if the loudspeaker is switched on by a gravity switch immediately the telephone handset is lifted from a base set housing the loudspeaker. An acoustic feedback path is produced between the telephone microphone transmitter and the loudspeaker so as to complete a feedback loop extending around the loudspeaker circuit. The acoustic feedback can produce howling at the loudspeaker, this phenomenon being well-known and termed the Larsen effect. As the microphone transmitter is moved away from the loudspeaker, the feedback loop is weakened and howling dies out allowing the system to be used normally. However, the initial noise is subjectively unpleasant.
2. Description of the Prior Art
Several proposals have been made for overcoming this problem. For example, the feedback loop can be made very lossy by lowering the gain of an amplifier controlling the loudspeaker input. This can, however result in an inadequate receive level for the listening group.
Alternatively a manually operable switch can be mounted on the base set. Unfortunately such a switch can easily be forgotten. If this occurs at the beginning of a call, information to the listening parties is lost; if at the end, howling results immediately prior to call termination as the handset is restored to the base set.
Another possible solution is the insertion of a time delay between the handset being lifted from the gravity switch and the loudspeaker being switched on. However the problem of howling when clearing down remains. In addition, the handset, when picked up, may not be moved directly away from the loudspeaker.
More complex schemes requiring signal processing are known, for example, that described in U.S. Pat. No. 3,787,621. These schemes appear to be particularly adapted not for group listening terminals, but for systems in which reception is by a loudspeaker only, or systems in which the telephone receiver and the loudspeaker are operated exclusively of one another.